S-LEC Wide Gradient Film

Within automotive OEM lamination cells, S-LEC Wide Gradient Film is processed as a PVB interlayer between the outboard soda-lime glass lite and the inboard thin glass lite. The gradient axis runs vertically, with the densest tinted zone placed at the upper edge of the windshield. The colour transition is continuous along the cross-web axis; no abrupt boundary line is present. Lay-up operators align the film using screen-printed alignment marks before the glass enters a deairing nip roller or vacuum bag station. The glass is preheated to 40–60 °C before the film is combined. The stack then passes through a vacuum bag or nip-roller deairing unit. On industrial lines, the vacuum bag is drawn to -0.08 MPa and held for 15–30 min during cold deairing. The pre-laminate is then heated to 90–120 °C while vacuum is maintained. Autoclave cycles for PVB interlayers commonly run at 120–140 °C and 1.1–1.4 MPa for 60–120 min; the exact setpoint is tuned to glass thickness and furnace load. After the hold segment, the laminate is cooled under pressure until the core glass temperature falls below 40 °C. The gradient zone can create a differential viscosity response because the pigmented phase absorbs more infrared than the clear phase during radiant preheating. This produces a variation in PVB flow at the top of the windshield. To compensate, lamination cells frequently adjust the upper and lower infrared emitter banks independently. Edge squeeze-out in the shaded zone is measured after autoclaving; excess flow exceeding 2–3 mm beyond the glass edge is trimmed. Final inspection includes transmitted wavefront analysis, optical distortion mapping, and luminous transmittance measurement under ASTM D1003-21. Impact and fragmentation requirements are verified according to UN ECE R43 and FMVSS 205 for the intended vehicle market. The pummel adhesion of the tinted zone is monitored because pigment migration can affect glass–PVB bond strength; typical PVB pummel values are maintained between 3 and 8. Moisture ingress at the cut edge is controlled by edge deletion and by storing unprocessed film at 18–25 °C and 40–60 % RH. Film moisture above 0.5 wt% increases bubble defects before the autoclave heat soak.

On high-speed laminating lines, the most common failure modes in the gradient transition region are bubble clusters and micro-lines. These arise when the temperature differential between the shaded and clear zones exceeds roughly 5 °C during the radiant preheat stage. The darker band absorbs more emitted infrared than the surrounding clear glass, so a uniform emitter output produces a non-uniform PVB viscosity. Laminators respond by lowering the upper emitter intensity or by reducing line speed to 0.5–1.5 m/min during preheat. If the line uses a vacuum bag rather than nip rollers, the film is preheated in a forced-air oven at 40–60 °C for 10–20 min to produce uniform tack. The lay-up room must be free of particulates larger than 50 µm in the transition zone because such particles become visible as local distortion after autoclave. Final optical inspection tests for windshield Zone B luminous transmittance and visual distortion against the vehicle glazing specification. When the gradient band crosses the camera mounting bracket area, an additional optical mapping step with a transmitted wavefront sensor is commonly implemented.

Which Lamination Sequence Prevents Colour Bleeding at the Spandrel-to-Vision Transition?

Architectural fabricators using the wide gradient film in unitised curtain wall panels typically process larger formats than automotive cells. The film is cut such that the gradient boundary aligns with the mullion or spandrel line. The glass is often heat-strengthened or tempered float. The PVB interlayer is laid onto the outboard lite, then the inboard lite is lowered. Deairing on a flat vacuum bag line is performed at room temperature, then heated to 80–120 °C. The autoclave cycle in architectural lamination is frequently extended because larger panels have greater thermal mass. Typical conditions are 135–145 °C and 1.2–1.4 MPa with a soak of 90–150 min. If the autoclave temperature ramp is too aggressive, the tinted area can develop localised flow lines, especially where thick PVB meets a step edge or frit band. Ceramic frit is sometimes screen-printed along the perimeter to protect sealant adhesion; the gradient film should not terminate on the frit without an edge-deleted margin of at least 4 mm. The continuous shade transition is used to conceal spandrel insulation and mechanical fixings while maintaining vision-glazing luminous transmittance. When glass is exposed to asymmetric solar loading, the dark zone transfers more heat to the PVB, increasing plasticizer mobility and accelerating slight colour drift in the transition zone. Fabricators evaluate xenon-arc weathering under ISO 4892-2:2013 and measure colour change using CIELAB instrumentation. For exterior exposed spandrel conditions, published data for this specific configuration is limited; facade mock-ups are required. Post-lamination, the laminate is tested for boil performance per EN ISO 12543-2:2021 and impact per ANSI Z97.1-2015 or CPSC 16 CFR 1201.

Failure modes observed on flat-bed vacuum laminators include edge star clusters near the gradient boundary and soft edges when the film shifts during bag evacuation. A 0.5 °C/min heat ramp inside the autoclave reduces temperature lag between the top and bottom lite; faster ramps can result in a temperature difference across the glass thickness that exceeds 10 °C for panels thicker than 10 mm. Thick architectural laminates are often autoclaved with intermittent fan reversal to homogenise the internal atmosphere. After the cycle, the panel is cooled in the open autoclave until a surface temperature below 40 °C is reached; premature removal causes edge ripples at the transition line. Dimensional stability of the gradient band is checked by measuring light transmittance at defined points along a vertical line. Edge bubbles or haze formed after 2 h in boiling water indicate insufficient deairing or high film moisture.

Rail vehicle side windows assembled for regional and metro rolling stock integrate the gradient film to reduce solar gain in the upper aperture while preserving outward visibility in the lower band. The interlayer is often paired with tempered or heat-strengthened glass and a laminated safety glass construction. The laminate stack is assembled in a clean lay-up room; the gradient film is oriented with the shaded portion at the top. Unlike automotive glazing, rail windows are regularly replaced, so aftermarket laminators may use a vacuum bag rather than high-speed nip rollers. The vacuum bag is evacuated to -0.09 MPa, held for 20–35 min at ambient temperature, then heated under vacuum to 100–120 °C. Final autoclaving is performed at 130–140 °C and 1.2 MPa for 90–120 min. Pressure cooling continues until the laminate centre temperature is below 40 °C. Adhesion at the tinted zone is checked by compressive shear tests per EN 14449. Rail certification requires impact performance meeting EN 15152-2 and EN 12600. Optional fire performance under EN 45545-2 is not a property of the PVB film alone but of the full laminate and frame assembly. The continuous gradient reduces the visual mismatch between anti-glare films and clear areas when passengers look upward at signals. Edge quality is critical because rail vibration loads can propagate from mounting clips into the laminate edge; an edge deletion of at least 2 mm is common. Storage of cut blanks at 18–25 °C and 40–60 % RH before autoclave is required. Incoming inspection of the tinted zone transmittance uses a spectrophotometer calibrated to CIE Illuminant A. The fabricator records the cross-web position of the gradient transition for each roll because the transition position may vary across production lots. This position is critical when the window has a ceramic frit at the lower edge. The film is not pre-shrunk; instead, the cut blank is stored for 2–4 h at room temperature to relax after slitting. Autoclave pressure above 1.4 MPa is not used where an additional polymer liner is present because the liner may indent under the mounting clips. Certification testing includes impact, fragmentation, and optional fire performance; the gradient film must not reduce the luminous transmittance in the vision zone below the rail contract requirement.

Forced-Entry Laminates with a Continuous Shading Transition

Security glazing fabricators employ the wide gradient film in multi-ply configurations. The shaded PVB interlayer is combined with additional clear PVB interlayers and annealed or heat-strengthened glass lites. Total interlayer thickness may reach 2.28 mm to 4.56 mm depending on the attack level. The lamination sequence is altered because the gradient film is not placed directly against a polycarbonate layer without an intermediate clear PVB layer; differential plasticizer migration can cause local haze or microcracking. Lay-up is performed with the gradient axis aligned to the upper or lower zone depending on the visual shielding requirement. Vacuum bag deairing is slower for thick laminates; cold deairing may be extended to 30–45 min. Autoclave conditions for security PVB laminates are often at the upper end, around 140–145 °C and 1.3–1.4 MPa, with hold times of 120–180 min for thick glass stacks. Excessive temperature in the tinted zone can cause selective plasticizer evaporation, visible as microvoids along the transition line. Process engineers monitor edge bleed and boil stability. Forced-entry performance is tested according to EN 356 categories and UL 972 for glazing materials intended for burglary resistance. Impact safety classification follows EN 12600. Because the gradient zone changes visible light transmittance, inspectors measure light transmittance in both the clear and shaded portions during qualification. Published data for this specific gradient film in multi-ply security configurations is limited; fabricators should run comparative pummel and weathering coupons before production. The edge condition after autoclave is critical; exposed PVB at the glazing edge must be kept dry, and any moisture path from the mounting channel must be interrupted with a compatible edge sealant. The gradient zone should not be used as the sole visual barrier for forced-entry-rated doors without confirming the required test category for the full fenestration assembly.